This invention relates generally to stereo synthesizers and, more particularly, has reference to a new and improved method and apparatus for converting the monaural audio tracks of audiovisual programs into surround stereo signal which are mono-compatible and steerable and which are synchronized with the video portion of the program.
In early movies and television programs, all of the sound elements in the audio portion of the program (i.e., dialogue, music and effects) were combined into a composite monaural signal which was recorded onto a single optical sound track. On playback, the optical track was scanned by a reader which recovered the composite monaural signal and fed the signal into the input of a monaural sound system.
Later fllms, taking advantage of magnetic tape recording techniques, used magnetic sound tracks. These tracks often had less surface noise (e.g., clicks and pops) and less distortion than optical tracks, but they generally continued to employ a composite monaural signal which was designed to be played through a monaural sound system.
An audiovisual program with a monaural sound track tends to lack realism. The sound remains stationary despite the fact that the sound elements may be moving around in the visual field. Stereo sound is generally regarded as more realistic and more pleasing to the ear because the sound can be moved around and placed in the sound field where it appears in the video picture. For example, the sound of a siren can be moved from left-to-right in the sound field as a police car speeds across the screen.
It would be highly desirable to produce movies and other audiovisual programs with true stereo sound tracks. Unfortunately, many early attempts to record stereo movies were not entirely satisfactory. The microphone array used for recording was often heavy and caused shadows. Post-production and dialogue replacement was often difficult. The process tended to be expensive and there were certain technical difficulties in producing consistent stereo scene-to-scene.
The continuing desire for stereo sound led to the development of so-called stereo synthesizers. These devices were passive "boxes" which received the output from a monaural audio source and purported to convert the composite monaura signal into a pseudo-stereo signal.
Conventional synthesizers fell into three general categories. The first used a comb filter to separate the monaural signal into alternating frequency bands and then placed the alternate bands into respective left and right channels. The second category used a time delay in which the monaural signal was separated into two channels with one of the channels being delayed by some time period. The third category combined a time delay and a comb filter.
These types of stereo synthesizers produced a stationary sound field in which the monaural sound was simply spread out in some fixed manner. The listener became accustomed to this fixed field and did not perceive any of the left-to-right or front-to-back movement of a sound which is characteristic of a stereo system.
Delay-type synthesizers also had a tendency to produce an echo in the audio program when the synthesizer channels were mixed together This could be a problem in applications such as television broadcasting and home video where it is often desirable to restore the original monaural signal for playback through the monaural sound system of a conventional television receiver.
Stereo synthesizers and other types of devices which alter audio signals have been known for a number of years, and by way of example, several forms of such devices can be found in U.S. Pat. Nos. 4,489,439 (Scholz et al.), 3,670,106 (Orban), 4,188,504 (Kasuga et al.), 4,394,536 (Schima et al.), 3,217,080 (Clark) and 4,329,544 (Yamada).
There was recently a proposal for a new type of television sound system in which mono dialogue, mono music and panned effects were used to simulate a stereo sound. The system had some steering compatibility, i.e., the ability to move a sound around and place it in the sound field where it belongs, but the system operated with a multitrack audio source having separate monaural tracks for dialogue, music and effects. This "DME" source created problems of compatibility with the great numbers of audio programs which used a composite sound track. Moreover, the system left considerable room for improvement in creating convincing stereo-like sound which the ear would perceive.
When a stereo synthesizer is used with an audiovisual program, it is obviously desirable to produce a stereo sound which is well synchronized with the video program. The sound elements should change and move throughout the sound field as the corresponding visual elements change and move throughout the video field. Existing systems have not been entirely satisfactory in this respect. Passive stereo synthesizers derive sound fields from monaural audio signals which contain little or no video information. Certain active stereo synthesizers have accepted user input of video information but they operated manually. The user had to turn dials or the like to effect changes in the audio signals while the video program was being run in real time. With such a system, it was difficult to accurately synchronize the audio signal with the video program, particularly where the video program required rapid or complex changes in the sounds.
Accordingly, a need exists for a stereo synthesizer which can produce a steerable surround stereo signal from a composite or separate monaural sound tracks used in audiovisual programs, which can automatically maneuver the sound signal left-to-right or front-to-back in the sound field in a manner which is well-synchronized with the movement of the corresponding visual elements in the program, and which can restore the program's original monaural signal for broadcast or playback through a conventional monaural sound system. The present invention fulfills all of these needs.